Go to the Dark Aether and head for the crashed plane. Here, you will find a small box beneath the floating plane wreckage. You need to do the following steps as quick as possible, as there is a time limit:
Melee the small box will give you a fuse. Holding the fuse, make your way to the Weapons Labs in the underground facility and place it in the Plasma Cutter (found across of the perk machine).
Now return to the normal world and go to the water pond. Here, you will find a small box at the old truck and you will find the D.I.E. Thermophasic upgrade when interacting with it.